Ma'am- varies if people will find it polite or inapropriate. In the US children might call their female relatives or teachers this, or in the service industry a worker may say this to an older female customer. However it could be taken to be too friendly or assuming of the woman's age. I wouldn't use this in a work email.

Miss - unmarried woman

Ms - any age/marital status woman

Mrs- married woman

Madam - perhaps a bit formal or awkward sounding for business; again may be heard in hospitality to a customer

Dear Mr. X or Ms. Y is the best choice for a workplace.

Saying 'Dear Sir/Ms' (together) however, may make the email sound generic - if you dont know their gender & surname, perhaps best to leave it as

'To whom it may concern..' :)

Hope this helps!

Kelly,

Interesting to contrast our views on "Madam". Perhaps generation comes into play here, but I would never consided Madam too formal for a business context. It's always best to use a name if you have one, of course.

I find "To whom it may concern" very cold and I never use it, however current UK produced English textbooks teach it.

I said that ma'am is not used in British English, but I think actually it's the correct way to address the Queen after you have been introduced. I think you have to say it to rhyme with "harm" not with "ham". Needless to say I've never had the occasion to use it in this way.

To address someone whose name you do not know in formal letters/emails,  you can write:

Dear Sir
Dear Madam
Dear Sir/Madam

Or if it gets confusing , you can simply write :

To whom it may concern

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To address someone in informal letters/emails for your friends or colleagues , you can write many things:

Dear Sarah
Dear friends/Dear colleagues/Dear editors ...  ( if you are sending one email to several recipients ) 

Hope that could help! :)

In a formal email, Dear Sir or Madam is fine.

You could use Dear Mr/Ms if you also have a surname but do not know if the person was male or female, but in that case I would probably just use Dear Sir or Madam.

Using Ma'am is not done in a British English.